The field of invention is cardiopulmonary exercise testing.
Exercise capacity is the best predictor of the future health of patients who suffer coronary artery disease or who have suffered heart failure. These diseases are the leading causes of hospitalization and mortality in the United States. Thus, exercise testing is a basic tool of clinicians, and is widely used. Analysis of expired gas during exercise is commonly known as cardiopulmonary exercise testing ("CPX") or metabolic exercise testing, and is often referred to as exercise testing with gas analysis. CPX has been considered by many clinicians to be difficult and expensive to perform, and because of this many clinicians have foregone CPX in favor of less accurate tests that merely estimate the measurements made directly by CPX. Such tests typically require the patient to exercise under steady state conditions--that is, a constant work level--for a fixed period of time, at the end of which the patient's heart rate, breathing rate, and oxygen consumption ideally plateau out to constant levels. The constant work level is then increased to a higher constant work level for a fixed time, and the patient's measurements are again expected to plateau out at the end of that time. This process may be repeated several times.
The measurement VO.sub.2 is the patient's oxygen uptake; that is, the rate of oxygen consumption by a patient during an exercise test. This measurement is sometimes referred to in terms of Mets, which are multiples of resting VO.sub.2, assumed to be 3.5 milliliters per kilogram per minute. Peak VO.sub.2, which is the maximum rate of oxygen consumption by a patient during an exercise test, is a good objective measurement of a patient's aerobic exercise capacity, and usually reflects cardiac function. As commonly performed, exercise testing merely estimates peak VO.sub.2 from exercise duration on a treadmill, workload on a stationary bicycle or distance walked. Such estimates may be substantially influenced by factors other than the patient's medical condition, however, such as the degree of patient effort and motivation, the degree of patient familiarity with the test equipment (sometimes referred to as the training effect); the disparity between expected oxygen requirements and actual oxygen uptake widens as heart disease worsens. This gap is filled by anaerobic processes, which result in the production of lactic acid when carbohydrate is metabolized in the absence of oxygen uptake. This leads to errors when VO.sub.2 is estimated by assuming the whole exercise process is fueled by aerobic metabolism.
VCO.sub.2 is the rate of carbon dioxide production by a patient during exercise. VCO.sub.2 relative to VO.sub.2 is influenced by which substrate is metabolized (fat vs. carbohydrate) and whether anaerobic processes and lactic acid production occur. Therefore, VCO.sub.2 cannot be estimated. VE (minute ventilation) is the volume of air breathed per minute by a patient, which varies proportionally to VCO.sub.2. VCO.sub.2 relative to VE is influenced by the presence of heart or lung disease. The calculation of VO.sub.2 and VCO.sub.2 by numerical integration of the product of expiratory airflow with O.sub.2 and CO.sub.2 concentrations over the duration of a breath is taught in the prior art.
Two different sources of error are commonly found in gas analysis equipment: delay time and response time. Delay time is the time taken for the physical transport of a gas sample from the mouth to the gas analyzers. On the other hand, response time, also known as rise time, is intrinsic to a gas analyzer. Response time is the time that elapses between exposure of a gas sample to a gas analyzer and an output signal from the gas analyzer achieving 67% of the full-scale signal that would correspond to the actual concentration of the gas. For example, if a gas sample containing carbon dioxide at a 10% concentration were exposed to a gas analyzer, the response time of that gas analyzer would be the time taken for that gas analyzer to output a signal indicating a 6.7% concentration of carbon dioxide. The errors introduced by the delay time and the response time prevent the accurate time synchronization of O.sub.2 and CO.sub.2 signals with separately-measured flow signals that do not experience delay time and response time errors, and thus prevent realtime measurement of VO.sub.2 and VCO.sub.2 and realtime calculation of derived parameters that depend on VO.sub.2 and VCO.sub.2, such as VE/VO.sub.2 and VE/VCO.sub.2.
Before calibrating a CPX system, it is often desirable to purge it of remnants of test gas or previous reference gas and ensure that ambient air is present in the system. This is essential for calibration, because if the system is not filled with ambient air before calibration, it will not be at a standard baseline state for the initiation of calibration.
Masks for collecting gas during exercise testing are known in the art, and may be used instead of the traditional mouthpiece and noseclip. However, tradeoffs are made between patient comfort during use, ease with which the operator can place the mask on the patient, and security of attachment to the patient. Typically masks which securely attach to the patient during exercise are difficult to put on the patient, and are uncomfortable; such discomfort can distract the patient during CPX and result in submaximal effort, or in early test termination due to patient discomfort.